Any weight loss / weight gain regime that doesn't consider, at the very least, thyroid hormones is bollox.

Steroidal sex hormones can play a role, particularly female sex hormones.

Allergies too.

i mean as far as we know they don't enable you to digest carboxymethylcellulose or anything like that (one of the many common additives on my suspect list)

it's not totally inconceivable; some commensal enteric bacteria can digest carboxymethylcellulose, and they generate heat when they do so, and at least when it's cold outside that's heat you don't have to generate by shivering or physical activity, and we have such a poor understanding of what affects the intestinal microbiome that it's actually possible that thyroid hormone levels are important, but there's no evidence suggesting that this is the case as far as i know

Thyroid hormones regulate metabolic rate, and have an effect on appetite.

Everything you eat effects the gut microbiome.

How could it be otherwise? Even putting any amount of even a nontoxic nondigestible substance in will have some effect due to displacing something that should be there instead, at least from a surface area interaction effect.

That might not always be clinically relevant, but it's still important to keep in mind in a clinical setting.

The idea that we don't fully understand, or that there is an absence of evidence in the face of the unknown, doesn't mean we should disregard.

Also, I argue strongly that there is almost always a significant, and generally untreated, psychogenic component to disease.

How could it be otherwise? Or bodies are basically not that different from other mammals, but our minds drive us to do the whackiest things.

Calories are an simplified approximation, where your body handles things in the same way as a fire does.

Calories are bunk in that there's a random function between the thermodynamics your body adheres to, and the thermodynamics the food adheres to. One with properties like the total calories your body has access to must be less than the total in, but for a specific food, or a specific meal, the total calories could be larger than the food in the meal, if it makes other calories easier to process, or even a negative, if it makes you puke.

Between the consumption of glucose and gasoline, there exists a spectrum of efficiency in the conversion of thermodynamic calories to useful bodily energy.

Take glucose for example. It's thermodynamic and metabolic calorie values are nearly equal (15.5MJ/kg or 3.7kcal/g). On the opposite end is gasoline, thermodynamically it's 46 MJ/kg but it's 0kcal/g if one were to ingest it. There simply isn't a metabolic pathway to extract energy from gasoline in human physiology.

I have no reason to doubt that the ratio of thermodynamic to metabolic efficiency varies amongst individuals, over time, and in the presence or absence of other compounds. When 9-14kcal/day supports the maintenance of 1kg of body mass in equilibrium, it's clear that, ceteris paribus[1], minute changes in metabolic efficiency can explain rather large differences in human mass.

1. Truly. Even/especially in the absence of differences in consumption.